MicroRNAs (miRNAs) are a phylogenetically conserved class of post-transcriptional regulators that are excellent candidates for finely tuning immune responses. These small (20-25 nucleotide) non-coding RNAs bind to target mRNAs by base-pairing to effect mRNA degradation or translational repression. The ability of microRNAs to shape host-virus interactions has been reviewed in Xiao, C., et al., Cell (2009) 136:26 and Skalsky, R. L., et al., Annu. Rev. Microbiol. (2010) 64:123. For example, miR-122 directly interacts with the hepatitis C viral genome to stimulate translation and aid viral growth, and cellular or viral-encoded microRNAs directly bind to viral mRNA transcripts to inhibit viral growth of HCV or promote latency of herpes virus and HIV. However, little is understood of the mechanisms underlying how host miRNAs shape antiviral resistance by controlling innate signaling pathways.
The present invention is based in part on our finding that miR-144 is a negative regulator of a module of antiviral interferon-induced genes controlled by TRAF6 and IRF7 which fine-tune the capacity of epithelial cells to restrict viral replication. miR-144 (and miR-451) are clustered on mouse chromosome 11, and are co-expressed as one pre-miR transcript in erythroid cells where they bind to unique sequences in target genes to play non-redundant roles in lineage differentiation (Papapetrou, E. P., et al., Stem cells (2010) 28:287; Dore, L. C., et al., Proc. Natl. Acad. Sci. USA (2008) 105:3333; and Fu, Y. F., et al., Blood (2009) 113:1340). It has been shown that constitutive expression levels of miRNAs provide a mechanism for maintaining mucosal homeostasis (Chassin, C., et al., Cell Host Microb (2010) 8:358).
The present invention is also based on the surprising finding that cultured cells that overexpress miR-144 offer greatly increased viral titers when then infected with virus. A variety of viral strains show this greatly enhanced production in such cells. This offers a much more convenient way to prepare viral vaccines, therefore, than methods currently employed which are quite expensive and cumbersome. Currently employed methods most commonly use fertilized eggs to culture the viruses. In this process, eggs containing embryos are infected with virus and incubated for 2-3 days before killing the embryo and physically removing the allantoic fluid. This procedure clearly is labor-intensive as well as awkward. These operations can be automated, but this requires expensive equipment.
Alternatively, cell lines such as Vero cells, MDCK cells and PER.C6® cells have been employed but these methods do not produce large quantities of virus and they, too are expensive. In general, the recovered viruses are processed by “splitting” or inactivating the harvested virus in a manner similar to the virus harvested from fertilized eggs. In an alternative to inactivating the viruses, the virus may have been attenuated by genetic modification. These standard techniques are less than satisfactory due to costs and inefficiency.